Ma! This time for Apple 4th generation and 4K TVs: the tvOS 12.1 (build 16J602) has just been released for everyone.
There is no clear information on what this update brings to Apple TV, plus general improvements and bug fixes. TvOS 12, released in mid-September, came with Dolby Atmos support and some new screensavers.
As always, the latest tvOS can be obtained from System Settings Software Update. If you paint any details about version 12.1, we will update this post.